A water absorbent resin is recently developed as a substance having high water absorbent property, and is widely used mainly for a disposable purpose, for example, as an absorbent body of hygiene product such as a disposable diaper and a sanitary napkin, an agriculture/horticulture water retaining agent, an industrial waterproofing agent, and the like. Various kinds of water absorbent resin exist, and a large number of monomers and hydrophilic polymers exist as raw materials for such water absorbent resins. Among them, a polyacrylic acid (salt)-based water absorbent resin using acrylic acid and/or salt thereof as a monomer is most abundantly manufactured in the industry due to its high water absorption performance.
The water absorbent resin is produced through many steps such as polymerization step, drying step, pulverizing step, classification step, and surface cross-linking step (Patent Literatures 1 to 3 and Non-Patent Literature 1). In accordance with having high water absorption performance of a disposable diaper as a main use, the water absorbent resin is also required to have various functions. Specifically, without being limited simply to the quantity of water absorption capacity, the water absorbent resin is required to have various physical properties including gel strength, water soluble components (Patent Literature 4), water absorbent speed, water absorption capacity under load (Patent Literature 5), liquid permeability, particle size distribution, urine resistance, microbial resistance, impact resistance, powder fluidity, deodorizing property, color resistance, low dust, or the like. For such reasons, various suggestions relating to a surface cross-linking technique, additives, modifications of a step for production, or the like have been made in addition to Patent Literatures 1 to 23 and Non-Patent Literature 1. Recently, in accordance with an increased use amount of the water absorbent resin in a disposable diaper (for example, 50% by weight or more), the liquid permeability is found to be a more important factor. In addition, there have been many methods or techniques suggested for improving liquid permeability under load or liquid permeability without load like SFC (Saline Flow Conductivity/Patent Literature 6) and GBP (Gel Bed Permeability/Patent Literatures 7 to 9).
Further, regarding the aforementioned physical properties, many combinations of plural parameters including liquid permeability have been suggested, and a technique for defining impact resistance (FI) (Patent Literature 10), a technique for defining water absorbent speed (FSR/Vortex) (Patent Literature 11), a technique for defining a product of liquid diffusion property (SFC) and a core absorption amount after 60 minutes (DA60) (Patent Literature 12) are known.
Further, as a method for improving liquid permeability like SFC and GBP, a technique for adding gypsum either before or during polymerization (Patent Literature 13), a technique for adding a spacer (Patent Literature 14), a technique for using 5 to 17 [mol/kg] of a nitrogen-containing polymer which has a protonizable nitrogen atom (Patent Literature 15), a technique for using polyamine and polyvalent metal ions or polyvalent anions (Patent Literature 16), a technique for coating a water absorbent resin at pH of lower than 6 with polyamine (Patent Literature 17), and a technique for using polyammonium carbonate (Patent Literature 18) are known. Furthermore, a technique for using polyamine at soluble components of 3% by weight or more and a technique for defining withdrawing index (WI) or gel strength are known (Patent Literatures 19 to 21). Furthermore, a technique for using polyvalent metal salts with controlling methoxyphenol as a polymerization inhibitor in polymerization for improving coloration and liquid permeability is known (Patent Literatures 22 and 23).
Furthermore, a technique for controlling holding time between a drying step and a pulverizing step (Patent Literature 24), a technique for controlling the ratio of returning a pulverized product to a pulverizing step as a technique focusing on a pulverizing step (Patent Literature 25), a technique for eliminating electric charges as a technique focusing on a classification step (Patent Literature 26), a technique for using a tapping material (Patent Literature 27), and a technique for controlling tension of a screen mesh (Patent Literature 28) are also techniques for improving liquid permeability.
It is also known that the liquid permeability tends to decrease as the particle diameter of a water absorbent resin decreases. For example, as the content of particle having a particle diameter of less than 150 μm (fine powder) increases, the liquid permeability is known to decrease (Patent Literature 29). Accordingly, various techniques for controlling particle diameter have been suggested until now.
For example, a technique for controlling particle diameter during a polymerization step like reverse phase suspension polymerization (Patent Literatures 30 and 31), a technique for controlling particle diameter during a step of micronizing water-containing gel-like crosslinked polymer (gel-crushing step) (Patent Literatures 32 to 34), a technique for controlling particle diameter or the like during a classification step (Patent Literatures 26 to 28 and 35 to 40), a techniques for granulation during a surface cross-linking step (preparing large particles by adhering fine powders or fine powders and target particles) (Patent Literatures 41 and 42), and a technique for removing fine powders with air flow during a cooling step after surface cross-linking (Patent Literature 43) have been also suggested.
Furthermore, as techniques focusing on a pulverizing step, a technique for performing pulverization after cooling a dried polymer (Patent Literature 44), a technique for performing pulverization after heating or warming a pulverizer (Patent Literature 45), a technique for reducing the ratio of particles other than target product in a pulverization product by pulverizing a dried polymer having high moisture content (Patent Literature 46 (in particular, FIGS. 1 to 4)), a technique for using a three-stage roll mill (Patent Literature 47), a technique for controlling holding time between a drying step and a pulverizing step (Patent Literature 48), and a technique for controlling the returning ratio of a pulverized product to a pulverizing step (Patent Literatures 25 and 49) have been also suggested. Furthermore, a method having different roll revolution speed for two rolls for pulverization by a roll mill is also known (Non-Patent Literature 1 (in particular, FIG. 3.8)).
Furthermore, as a technique relating to pulverization, a technique for removing non-dried products in a dried polymer before pulverization (Patent Literatures 50 to 52) and a technique for installing a magnetic separator between a top part and a bottom part of a roll mill (Patent Literature 53) have been also suggested.